1. Field of the Invention
The present invention relates to a network communication apparatus for securing establishment of call connection. In the following, basic terms used in the specification are described.    1. JT-H323 defines general regulations for exchanging multimedia information such as voice information, image information, and the like on an IP network. JT-H323 also defines H.323 faststart.    2. JT-H225.0 defines details of message signals for connecting calls used in H.323.    3. JT-H245 defines a procedure (Slow Start) after call connection based on JT-H225.0 is established.    4. JT-T38 defines real-time communication procedures of facsimile on the IP network.    5. H.245 tunneling is a method for exchanging H.245 signals on an H.323 session while eliminating the necessity of creating a dedicated session. Normally, when H.245 signals are exchanged between terminals, a TCP session dedicated to H.245 is connected other than the H.323 session and the exchange is performed on the session. Specifically, an H.323 signal referred to as “FACILITY” is used for tunneling. The FACILITY is an H.323 signal for functionality expansion along with SETUP, CONNECT, and the like. The FACILITY has a header common to H.323 signals and an area for functionality expansion. By having data based on the H.245 standard in the area for functionality expansion, it is possible to exchange H.245 signals on the H.323 “FACILITY”.
2. Description of the Related Art
Upon using real-time Internet facsimile, namely, IP-FAX, call connection procedures between communication terminals are standardized in JT-H323 and RFC3261. However, there is a nonnegligible difference of call connection procedures on the market depending on VOIP device vendors. For example, Cisco Systems, Inc. having a large share in the VoIP market supplies products that are not based on the standards, so that when products of other companies are solely based on the standards, call connection may not be established.
The problem of connectivity upon using IP-FAX results from specifications of VoIP devices on the market such that the VoIP devices are assumed to be connected to IP-Phones. In other words, in a call connection procedure, connection of a voice channel is established by priority and then connection of a facsimile communication channel is established upon request. However, call connection is not established for products that do not follow such a call connection procedure.
In particular, in some VoIP devices, although a connection signal “INVITE” is transmitted first in a call connection procedure based on RFC3261(SIP), when a T.38 capability is declared in the signal, the call is immediately terminated. In order to maintain connectivity to such devices, it is necessary to declare only a voice capability in the INVITE signal. Further, there is a demand for performing a general call connection operation based on the standards other than communication with devices of a specific vendor.
In patent document 1, a voice session and a facsimile data session are established at the same time and data exchange is enabled respectively, so that both sessions are separately used in accordance with capability of a target terminal. However, in some VoIP devices following the call connection procedure based on RFC3261(SIP), when the facsimile data session is established, the call is immediately terminated at that time, so that patent document 1 does not meet the above-mentioned demand.
Patent Document 1: Japanese Laid-Open Patent Application No. 2004-96326